Solving a Problem by Using a Diagram

Sketches and diagrams are sometimes useful in problem solving．For example,four runners are in a one-mile race:A,B,C,and D．Points are awarded only to the person finishing first or sec- ond．The first-place winner gets more points than the second-place winner．How many different arrangements of first-and second-place winner are possible?     

When is a Coalgebra a Generator?

Let C be a coalgebra over a field k. The aim of this paper is to study the following problem : (P) If C is a k-coalgebra such that C is a generator for the category of left comodules, is C a left quasi-co-Frobenius coalgebra ? The converse always holds. We show that if C has a finite coradical series, the answer is positive.     

Using a Table to Make a Generalization

A generalization is a statement that is true about many instances.The following story is aboutmaking a generalization.Francie lives in Phoenix.She has a telephone in her room.She pays for all long-distance callsshe makes.One Sunday she called her cousin Meg in San Diego.As they talked,Francie forgot     

When is a stochastic integral a time change of a diffusion?

We give necessary and sufficient conditions that a time change of ann-dimensional Ito stochastic integralX _t of the form

Draining a polygon—or—rolling a ball out of a polygon

We introduce the problem of draining water (or balls representing water drops) out of a punctured polygon (or a polyhedron) by rotating the shape. For 2D polygons, we obtain combinatorial bounds on the number of holes needed, both for arbitrary polygons and for special classes of polygons. We detail an $O(n^2\log n)$ algorithm that finds the minimum number of holes needed for a given polygon, and argue that the complexity remains polynomial for polyhedra in 3D. We make a start at characterizing the 1-drainable shapes, those that only need one hole.     

On a universal Σ-function over a tree

We obtain a sufficient condition for the absence of any universal ??-function in an admissible set (a hereditarily finite admissible set). We construct a tree T of height 4 such that no universal ??-function exists in the hereditarily finite admissible set ? $\mathbb{F} $ (T) over T.     

角a、a/2、2a终边位置的研究报告

(~责审余炯沛)a终边位置一象限二象限三象限一四象限一注释代数表示 Zk兀相似文献   

Exit Time of a Hyperbolic α-Stable Process from a Halfspace or a Ball

For a hyperbolic α-stable process in the hyperbolic space \(\mathbb {H}^{d}, d\ge 2\), we prove that the mean exit time from a halfspace \(H(a)=\{x_{d}>a\}\subset \mathbb {H}^{d} \) is equal to \(\mathbb {E}^{x} \tau _{H(a)} = c(\alpha , d) \delta ^{\alpha /2}_{H(a)} (x),\) where δ_D(x) is the (hyperbolic) distance of x to D^c. Based on this exact result we provide a sharp estimate of the mean exit time from a hyperbolic ball B(x₀,R) of radius R and center x₀: \(\mathbb {E}^{x}\tau _{B(x_{0},R)}\approx (\delta _{B(x_{0},R)}(x) \tanh R)^{\alpha /2}, x\in \mathbb {H}^{d}\). By usual isomorphism argument the same estimate holds in any other model of real hyperbolic space.     

Turán Number of the Family Consisting of a Blow-up of a Cycle and a Blow-up of a Star

Let F={H₁,...,H_k}(k> 1) be a family of graphs.The Turán number of the family F is the maximum number of edges in an n-vertex {H₁,...,H_k)-free graph,denoted by ex(n,F) or ex(n,{H₁,H₂,...,H_k}).The blow-up of a graph H is the graph obtained from H by replacing each edge in H by a clique of the same size where the new vertices of the cliques are all different.In this paper we determine the Turán number of the family cons...     

比较a/b与(a 2b)/(a b)的大小

设a,b∈R~ 且a>b,讨论并比较a/b与a 2b/a b的大小。原题中含有两个变元a、b,为使问题化繁为简,引进新的变元t,且令t=a/b,显然有t>l,则(a 2b)/(a b)=(t 2)/(t 1)>1,这样,要讨论的两个分式的大小比较就转化为讨论t与(t 2)/(t 1)的大小比较了。 1°设t=t 2/t 1,则有t~2 t=t 2t~2=2,由     

Draw a picture

You can understand every word in a problem yet still not be able to solve it immediately.Oneuseful strategy is to draw a picture.This is particularly helpful when the problem involves a gcometric figure.     

由√a2=｜a｜想到的

由绝对值的定义,公式√a2=｜a｜可表示为√a2=｜a｜=a a≥0,-a a〈0.这里a可表示一个数,也可以表一个式子.     

Recovering a Lamé kernel in a viscoelastic system

In the equation of viscoelasticity related to a geometric cylinder, we recover the spatial part p of a factorized Lamé kernel depending on two spatial sectional variables. More explicitly, we determine sufficient conditions on the data ensuring the (local) uniqueness of p and its continuous dependence on the data in suitable metrics.     

On a Singular Dirichlet Problem With a Convection Term

ＯｎａＳｉｎｇｕｌａｒＤｉｒｉｃｈｌｅｔＰｒｏｂｌｅｍｗｉｔｈａＣｏｎｖｅｃｔｉｏｎＴｅｒｍＺｈａｎｇＺｈｉｊｕｎ（张志军）（Ｄｅｐｔ．ｏｆＭａｔｈ．ａｎｄＩｎｆｏｒ．Ｓｃｉｅｎｃｅ，ＹａｎｔａｉＵｎｉｖ．，Ｙａｎｔａｉ，Ｓｈａｎｇｄｏｎｇ，２６４０...     

Modelling ‘contracts’ between a terrorist group and a government in a sequential game

In this paper, we apply a sequential game to study the possibility of ‘contracts’ (or at least mutually beneficial arrangements) between a government and a terrorist group. We find equilibrium solutions for complete and incomplete information models, where the government defends and/or provides positive rent, and the terrorist group attacks. We also study the sensitivities of equilibria as a function of both players’ target valuations and preferences for rent. The contract option, if successful, may achieve (partial) attack deterrence, and significantly increase the payoffs not only for the government, but also for some types of terrorist groups. Our work thus provides some novel insights in combating terrorism.     

Hopf bifurcations in a predator–prey system with a discrete delay and a distributed delay

A delayed Lotka–Volterra two-species predator–prey system with discrete hunting delay and distributed maturation delay for the predator population described by an integral with a strong delay kernel is considered. By linearizing the system at the positive equilibrium and analyzing the associated characteristic equation, the asymptotic stability of the positive equilibrium is investigated and Hopf bifurcations are demonstrated. It is found that under suitable conditions on the parameters the positive equilibrium is asymptotically stable when the hunting delay is less than a certain critical value and unstable when the hunting delay is greater than this critical value. Meanwhile, according to the Hopf bifurcation theorem for functional differential equations (FDEs), we find that the system can also undergo a Hopf bifurcation of nonconstant periodic solution at the positive equilibrium when the hunting delay crosses through a sequence of critical values. In particular, by applying the normal form theory and the center manifold reduction for FDEs, an explicit algorithm determining the direction of Hopf bifurcations and the stability of bifurcating periodic solutions occurring through Hopf bifurcations is given. Finally, to verify our theoretical predictions, some numerical simulations are also included at the end of this paper.     

How often can a finite group be realized as a Galois group over a field?

Let G be any finite group and any class of fields. By we denote the minimal number of realizations of G as a Galois group over some field from the class . For G abelian and the class of algebraic extensions of ℚ we give an explicit formula for . Similarly we treat the case of an abelian p-group G and the class which is conjectured to be the class of all fields of characteristic ≠p for which the Galois group of the maximal p-extension is finitely generated. For non-abelian groups G we offer a variety of sporadic results. Received: 27 October 1998 / Revised version: 3 February 1999     

The flow of a combustible mixture with a Chapman–Jouguet detonation wave around a cone

The flow around a circular cone under conditions of Chapman–Jouguet (CJ) self-sustaining detonation is investigated in the classical formulation of an infinitely thin detonation wave (DW) in an inviscid and non-heat conducting combustible mixture. Such conditions for flow around a cone are remarkable in several respects. In 1959, Chernyi and Kvashnina had already shown that, for supersonic flows of a combustible mixture around a cone, CJ detonation, as in the case of a wedge, is not only possible for a strictly fixed cone angle (the “CJ” angle) but also for angles smaller than this (including a zero angle, that is, when there is no cone). In the case of flow around a wedge with an angle that is less than the corresponding CJ angle, a centred rarefaction wave that turns the supersonic flow in the required direction borders on the CJ DW. In the case of a cone, a conical rarefaction flow also borders on the CJ DW. However, if, in the plane case, a uniform supersonic flow adjoins the centred rarefaction wave along its boundary C⁺-characteristic then the conical rarefaction flow is bounded by a conical shock wave (SW), bordered up to the surface of the cone by a conical compression flow. Only for a zero cone angle does the SW degenerate into the C⁺-characteristic and the conical compression flow degenerates into a uniform supersonic flow. The configuration obtained in the general case became the first example of a self-similar solution with two SW of “one family” (the first of them is the DW) diverging from a single point. The results of calculations carried out in this paper with the construction of streamlines and the characteristics of the two families give a fairly complete representation of the above mentioned features of the flows considered.